1. Field of the Invention
The present invention relates to a rear projection screen for use as the display screen of a projection-type television. More particularly, the present invention relates to a Fresnel lens sheet useful in allowing imaging light obliquely projected on the lens sheet from the light-source (rear) side to emerge from the lens sheet toward the viewing side in order to display an image, and it also relates to a rear projection screen including such a Fresnel lens sheet.
2. Description of Related Art
To achieve projection (enlargement projection) of imaging light emitted from a light source, a rear projection screen is placed in a projection-type television as its display screen. Usually used for such a rear projection screen is a combination of a Fresnel lens element that refracts imaging light projected on the element from a light source and allows the refracted light to emerge from the element toward the viewing side as rays almost parallel to each other, and a diffusion element such as lenticular lenses that scatter the imaging light toward those viewers who are watching the screen from wide-ranged directions.
A projection-type television provided with a CRT-type light source, in which different CRT's are used to project rays of the three primary colors, as well as one provided with a single-lens-type light source using an LCD (liquid crystal device) or DLP (digital light processing), have come to be used as the above-described projection-type television.
Of these projection-type televisions, the projection-type television provided with a CRT-type light source has usually been such that imaging light is projected, from the light-source side of the rear projection screen, on the central part of the rear projection screen almost perpendicularly to it. Also in the projection-type television provided with a single-lens-type light source, imaging light has usually been projected from the light-source side of the rear projection screen on the central part of the rear projection screen almost perpendicularly to it. Namely, as shown in FIG. 9, a projection-type television of either type is so designed that a mirror 13 reflects imaging light 10 emitted from a light source 12 to project the reflected light on the central part of a rear projection screen 3 almost perpendicularly to it.
However, as long as the manner in which imaging light is projected is as described above, even the projection-type television provided with a single-lens-type light source requires a depth nearly equal to that of conventional one provided with a CRT-type light source, and thus has the problem that its cabinet 14′ cannot be made thin, as can be understood from FIG. 9.
Under these circumstances, a projection-type television, in which imaging light emitted from a light source is obliquely projected on a rear projection screen from its light source side, has been proposed in order to obtain a projection-type television much thinner than conventional ones without impairing image display performance.
In such a projection-type television, a group of prisms with triangular cross-sections (a total reflection Fresnel lens) are provided on the light-source-side surface of the rear projection screen as an optical means of collecting imaging light obliquely projected on the rear projection screen so that the imaging light projected is refracted at the first face of each prism and is then totally reflected at the second face of the prism, thereby allowing the reflected light to emerge from the viewing-side surface of the rear projection screen.
In a Fresnel lens sheet having such a total reflection Fresnel lens, since imaging light emitted from a light source is obliquely projected on the Fresnel lens sheet, the imaging light is to enter each prism at an angle of incidence that varies depending on the position of the prism located between the lower and upper edges of the Fresnel lens sheet.
Specifically, as shown in FIG. 7(a), in the part of a Fresnel lens sheet 15 on the side distant from a light source (e.g., the part of the Fresnel lens sheet 15 on the side distant from the Fresnel center, in the case where the prisms are concentrically formed around the Fresnel center that is situated not on the sheet surface), in which the angle of incidence θ1 at which imaging light 10 is incident on the Fresnel lens sheet 15 (the angle between imaging light 10 and the normal to the surface of the Fresnel lens sheet 15) is larger, the imaging light 10 is to enter each prism 43 from its first face 44, but only from the vicinity of the apex 41 of the prism 43. For this reason, all of the rays of the imaging light 10 that have entered each prism 43 from its first face 44 are totally reflected at the second face 45 of the prism 43, and all of the totally reflected rays emerge, as usable light 10A, from the viewing-side surface 48 of the Fresnel lens sheet 15.
On the contrary, as shown in FIG. 7(b), in the part of the Fresnel lens sheet 15 on the side close to the light source (e.g., the part of the Fresnel lens sheet 15 on the side close to the Fresnel center, in the case where the prisms are concentrically formed around the Fresnel center that is situated not on the sheet surface), in which the angle of incidence θ1 at which the imaging light 10 is incident on the Fresnel lens sheet 15 is smaller, the imaging light 10 is to enter each prism 43 from a wide area of its first face 44, covering from the apex 41 to the bottom 42. For this reason, of the imaging light 10, those rays that have entered each prism 43 from its first face 44, but from the vicinity of its apex 41 are refracted at the first face 44 and then totally reflected at the second face 45; these rays are to emerge from the viewing-side surface 48 as usable light 10A. On the other hand, those rays that have entered each prism 43 from its first face 44, but from the vicinity of its bottom 42 are refracted at the first face 44 and then pass through the base, without being totally reflected at the second face 45, to finally become stray light 10B.
As shown in FIG. 8, after being totally reflected at the viewing-side surface 48, the stray light 10B thus produced passes through a plurality of the first prisms 43 on the light-source side, while repeating incidence and emergence, and finally emerges from the viewing-side surface 48. The direction in which the stray light 10B thus emerges from the viewing-side surface 48 is almost the same as that in which the usable light 10A emerges from the viewing-side surface 48, so that such troubles as the production of double images (ghosts) are caused.
A variety of methods as stated below have so far been proposed as methods for eliminating such problems as the production of double images (ghosts) due to stray light as described above.
Specifically, such a method has been proposed that the grooves between Fresnel lens prisms are filled with a light-diffusing layer (see Japanese Laid-Open Patent Publication No. 113131/1987). In this method, it is necessary to form a light-diffusing layer after forming a Fresnel lens, so that the production process according to this method is complicated. In addition, it is not easy to control the amount of diffusion of light.
Another method that has been proposed is that a light-absorbing layer is partly formed on Fresnel lens prisms (see Japanese Laid-Open Patent Publications No. 32528/1988, No. 37336/1988 and No. 72634/1993). In this method, a light-absorbing layer is formed not entirely but only partly on the prisms. This method is thus at a disadvantage in that the formation of the light-absorbing layer is difficult.
A further method proposed is that a light-absorbing layer is formed on the viewing-side surface of a Fresnel lens sheet, but only on those parts of the surface through which imaging light does not pass (see Japanese Laid-Open Patent Publications No. 30835/1988, No. 37337/1988, No. 139331/1988 and No. 72634/1993). In this method, it is necessary to adjust the position of the light-absorbing layer to be formed on the viewing side, to the position of the prisms formed on the light-source side. This method is thus at a disadvantage in that the formation of the light-absorbing layer is not easy.
A still further method proposed is that the faces of Fresnel lens prisms from which imaging light enters the prisms are curved (see Japanese Laid-Open Patent Publication No. 254941/1986). However, from the viewpoint of production, it is not easy to vary the curvature of the curved face of a prism, i.e., a plane of incidence, depending on the position of the Fresnel lens prism; it is thus difficult to uniformly display an image.
A still further method proposed is that a light-diffusing layer is additionally formed entirely on the viewing-side surfaces of Fresnel lens prisms. This method has the problem that not only stray light but also imaging light is diffused to make the displayed image blurred.